Chlorinated alkenes, particularly TCE, were utilized in the past in a number of various chemical processes carried out by different industries. TCE was generally used for metal degreasing, solvent extraction, dry cleaning, fumigation, and as a chemical intermediate. TCE is a heavy, stable, toxic liquid with a chloroform aroma which is slightly soluble in water and soluble in most common organic solvents. TCE has a boiling point of 87.degree. C. TCE has been recognized as a hazardous organic contaminant in various aqueous liquids, especially groundwater. Groundwater is generally defined as all subsurface water, especially that part that is in the zone of saturation. The zone of saturation is generally defined as a subsurface zone in which water fills the interstices and is under pressure greater than atmospheric pressure. The presence of TCE in groundwater can adversely affect its potability, as well as the cultivability of the surrounding soil.
Possible solutions to this problem have included several approaches, two of which are steam stripping and biodegradation. Steam stripping, also referred to as steam distillation, is a process in which vaporization of the volatile constituents of a liquid mixture, in this case chlorinated alkenes, takes place by introducing steam directly into the charge. Steam used in this manner is known as open steam. Biodegradation on the other hand, covers a variety of different processes whereby living organisms are employed to perform a biochemical transformation on a substrate such that the substrate is converted into a desirable intermediate or end product. What is needed is a method and apparatus for removing organic contaminants from groundwater by integrating steam stripping with biodegradation, thus overcoming several deficiencies that occur when either steam stripping or biodegradation is used alone.
Steam stripping of contaminated groundwater produces an effluent waste stream which contains the organic contaminants, which will require further treatment and disposal. The present invention provides for an efficient on-site treatment technology that will remove the organic contaminants and will not produce any further major waste streams.
Aerobic biodegradation of organic contaminants is impeded by iron and other minerals in groundwater that precipitate onto the biofilms and interfere with biodegradation. Previously, methods such as ion exchange and demineralization were used to remove minerals prior to biodegradation. However, these methods create a new hazardous waste stream which would require further treatment and disposal. The present invention excludes these minerals and other dissolved solids from the biodegradation portion of the process without the need for explicit removal and handling.
Biodegradation is a relatively slow process at the low concentration of organic contaminants typically found in groundwater. Previously, all the contaminated groundwater was fed directly to the bioreactor without the benefit of a concentration step. The present invention takes advantage of the natural concentration step in steam stripping to provide a more concentrated feed stream to the bioreactor, which leads to more efficient biodegradation kinetics. Additionally, this concentration step reduces the volume of water to be treated in the bioreactor, which in turn allows for a substantially smaller bioreactor column. Consequently, the net capacity of the bioreactor is increased by approximately 20 fold. For example, a flow of 100 gallons per minute (gpm) of groundwater can be treated by a bioreactor that handles only approximately 5 gpm. Also, the overhead condensate produced by the steam stripping is treated via biodegradation, a technology that produces no significant further waste streams when coupled to a steam stripper.
Finally, biodegradation is relatively inefficient in producing the very low effluent concentration of organic contaminants necessary for discharge of the treated water back to the environment. Previously, the bioreactor effluent had to undergo further treatment prior to discharge to the environment. The present invention provides for the recycle of the bioreactor effluent back to the steam stripper, such that it is not necessary to achieve a high quality effluent from the bioreactor.